What are some techniques/method that can be used to train and detect objects like cars and humans?
The terms you are looking for are deeplearning and convolutional neural networks for object detection. Google responds well to these terms.
From academical point of view you can start from:
Single shot multibox detector: https://arxiv.org/pdf/1512.02325v5.pdf
Or Faster-RCNN:
https://arxiv.org/pdf/1506.01497.pdf
These are not simple architectures and there are many improvements to them but these give you an idea of the current state of the art methods.
There are many implementations of both of these networks in deeplearning libraries for python (eg in Tensorflow, PyTorch).